Biological determinism, also called biologism or biodeterminism, the idea that most human characteristics, physical and mental, are determined at conception by hereditary factors passed from parent to offspring. Although all human traits ultimately are based in a material nature (e.g., memorizing a poem involves changing molecular configurations at synapses, where nerve cells interact), the term biological determinism has come to imply a rigid causation largely unaffected by environmental factors. Prior to the 20th century and the rediscovery of Austrian botanist Gregor Mendel’s work on heredity, a wide variety of factors were believed to influence hereditary traits. For example, environmental agents were thought to act directly on the mother’s or father’s germ cells (eggs or sperm, respectively) or indirectly on the fetus via the mother during pregnancy. After the rediscovery of Mendel’s work, theories of biological determinism became increasingly formulated in terms of the then new science of genetics. Thus, biological determinism became synonymous with genetic determinism, though some researchers later considered the two to be distinct.

Early theories and applications

In the 18th and 19th centuries, theories of biological determinism were based on vague, often highly controversial ideas about the nature of heredity. Since the concepts and tools were not available during that period to study heredity directly, biologists and anthropologists measured physical features of humans, trying to associate mental and personality traits with anatomical (and occasionally physiological) features, such as facial angle (angle of slope of the face from chin to forehead) or cranial index (ratio of lateral to vertical circumference of the head). Certain physical features, such as high cheekbones or a prominent eyebrow ridge, were often said to be indicative of criminal tendencies. With the growing acceptance of Mendelian genetics in the first half of the 20th century, most theories of biological determinism viewed undesirable traits as originating in defective genes. With the revolution in molecular genetics during the second half of the century, defective genes became identified with altered sequences of the molecule of heredity, deoxyribonucleic acid (DNA).

For much of its history, biological determinism was applied to what were widely perceived to be negative traits. Examples included physical traits such as cleft palate, clubfoot, dwarfism, and gigantism as well as social and psychological conditions such as criminality, feeblemindedness, pauperism, shiftlessness, promiscuity, bipolar disorder, and hyperactivity. Whereas many researchers agreed that physical defects likely arise from genetic anomalies, the claim that all psychological disorders and socially unacceptable behaviours are inherited was controversial. That was partly due to the difficulty of obtaining rigorous data about the genetics of such traits. However, it was also due to an increasing knowledge of the abilities of various factors, such as chemicals in the environment, to interact with genetic elements. Teasing apart the genetic and environmental causes of psychological and behavioral conditions remains an exceptionally challenging task.

The eugenics movement

One of the most prominent movements to apply genetics to understanding social and personality traits was the eugenics movement, which originated in the late 19th century. Eugenics was coined in 1883 by British explorer and naturalist Francis Galton, who was influenced by the theory of natural selection developed by his cousin, Charles Darwin. Galton used the term to refer to “more suitable races,” or essentially those individuals who were well born. He argued for planned breeding among the “good” of the human population along with various methods to discourage or prevent breeding among defective individuals. It was the belief of eugenicists such as Galton, British statistician Karl Pearson, and American zoologist Charles B. Davenport that most social problems were due to the accumulation of genetic defects, which were producing an increasingly disabled, or “degenerate,” population. Eugenicists believed that society was deteriorating through the increased reproduction of the disabled, particularly the mentally disabled. Various forms of inherited mental disability were said to be the root cause of social problems as varied as crime, alcoholism, and pauperism (in all cases, it was claimed that low mental ability led to an inability to cope in a complex society, resulting in a turn to antisocial behaviours).

Using IQ tests developed in the 1920s and ’30s, eugenicists proceeded to rank people and place them in categories based on quantitative scores; the categories ranged from normal to high-grade moron, idiot, and imbecile. Individuals with slightly below average IQ scores typically were ranked as genetically disabled, even though they were not actually disabled at all; many, rather, were disadvantaged. In the absence of genetic testing, little sound evidence could be provided in support of the notion that such cases were genetically determined.

Sterilization laws were introduced in the 1920s in the United States and in the 1930s in Germany. More than half of U.S. states eventually adopted sterilization laws, which were aimed primarily at compulsory sterilization of those deemed to be genetically unfit in state and federal institutions, such as mental hospitals, asylums, and prisons. In the early 1970s it was revealed that thousands of people had been subjected to involuntary sterilization in the United States. Many more had experienced the same in Germany and other countries.

Multifaceted diseases

One of the major consequences of widespread belief in biological determinism is the underlying assumption that if a trait or condition is genetic, it cannot be changed. However, the relationship between genotype (the actual genes an individual inherits) and phenotype (what traits are observable) is complex. For example, cystic fibrosis (CF) is a multifaceted disease that is present in about 1 in every 2,000 live births of individuals of European ancestry. The disease is recessive, meaning that in order for it to show up phenotypically, the individual must inherit the defective gene, known as CFTR, from both parents. More than 1,000 mutation sites have been identified in CFTR, and most have been related to different manifestations of the disease. However, individuals with the same genotype can show remarkably different phenotypes. Some will show early onset, others later onset; in some the kidney is most afflicted, whereas in others it is the lungs. In some individuals with the most common mutation the effects are severe, whereas in others they are mild to nonexistent. Although the reasons for those differences are not understood, their existence suggests that both genetic background and environmental factors (such as diet) play important roles. In other words, genes are not destiny, particularly when the genetic basis of a condition is unclear or circumstantial but also even in cases where the genetic basis of a disability can be well understood, such as in cystic fibrosis.

With modern genomics (the science of understanding complex genetic interactions at the molecular and biochemical levels), unique opportunities have emerged concerning the treatment of genetically based disabilities, such as type I diabetes, cystic fibrosis, and sickle-cell anemia. Those opportunities have centred primarily on gene therapy, in which a functional gene is introduced into the genome to repair the defect, and pharmacological intervention, involving drugs that can carry out the normal biochemical function of the defective gene.

Influence on disability

Social attitudes about what constitutes a disability, and how economic and social resources are to be allocated to deal with disabilities, change over time. In hard economic times the disabled are often written off as “too expensive,” a trend often justified on the basis of genetic determinism (whether scientifically valid or not). Arguments for biological determinism have long been employed more to restrict than to expand human potential.